1. Technical Field
The present invention relates to an optical position measuring system for generating position-dependent scanning signals, which in particular provides scanning signals free of harmonics and includes a measuring graduation and a scanning unit, which are arranged to be movable in relation to each other in a measuring direction.
2. Background Information
Known optical position measuring systems for generating scanning signals in the form of periodic incremental signals customarily include a measuring graduation, as well as a scanning unit, which is arranged to be movable in a measuring direction relative to the measuring graduation. In turn, two objects, for example parts of a machine, whose relative position is to be determined by the position measuring system, are connected with the measuring graduation and the scanning unit. The generated scanning signals are fed to an electronic follow-up device for further processing, for example a numeric machine tool control or a servo drive control.
An arrangement of graduation areas of different optical properties, which is periodic in the measuring direction, is provided on the side of the linear or rotary measuring graduation. In the case of a transmitted light system, transparent and opaque graduation areas are here alternatingly arranged on the side of the measuring graduation, in the case of an incident light system highly reflective graduation areas and low-reflecting graduation areas, etc. For generating the scanning signals, the scanning unit includes at least a light source and an opto-electronic detector arrangement. Depending on the optical scanning principle employed, one or several scanning graduations can furthermore be arranged in the scanning beam path.
As a rule, the analog scanning signals generated in this way do not have an ideal signal form as a result of various influences, i.e. customarily no sinusoidal scanning signals exist. For example, inexactitude of the measuring graduation used, fluctuations in the scanning distance, or resultant diffraction effects are considered to be the cause of such deviations. Therefore the results are undesired harmonic portions in the scanning signals. However, signals which are free of harmonics to the greatest possible extent and ideally have a sinusoidal shape are desirable for error-free further processing of the scanning signals. This applies in particular if a signal interpolation is required in the position measuring system or the electronic follow-up device, by which the signal period provided by the scanning signals is again electronically sub-divided.
A multitude of solutions for reducing the harmonic content have already become known. The various attempts for harmonic filtering envision, for example, actions on the part of a scanning graduation or in connection with the measuring graduation. The respective graduations are modified in a suitable manner in order to minimize the undesired harmonic portions of the generated scanning signals. This can be done by a modification of the periodic arrangement of the graduation structures and/or by a suitable geometry of the graduation structures. In this connection the publications GB 2 116 313 A or EP 541 827 B1 are to be mentioned as examples.
Furthermore, a position measuring system is known from DE 1 282 988, wherein certain measures for harmonic filtering are being taken in connection with the measuring graduation, as well as in connection with the scanning graduation.
Various possibilities are furthermore known for reducing the harmonic content of the scanning signals by taking action in connection with the detector arrangement used. In this connection reference is made to EP 250 711 A2 or EP 714 015 B1. In these publications it is proposed to perform a defined harmonic filtering in the detector arrangement by a suitable arrangement and dimensioning of the individual detector elements. Undesired harmonic portions can be very efficiently eliminated from the scanning signals by such steps in connection with the detector elements. However, it is problematical here that respectively different embodiments or designs of the detector arrangement are required for different scanning configurations. If now such detector arrangements are intended to be employed in an integrated form in so-called OPTO-ASICs in connection with the scanning unit, a significant outlay regarding necessary changes in the detector arrangement then results in case of possibly required changes because of different filtering requirements. However, in case of using OPTO-ASICs it is basically desirable for them to be available as unchanged as possible for diverse applications.